Is The Majority Of Human DNA Really 'Junk'?

A new study examining fertility rates has estimated that around 75% of the human genome is non-functional - acting as a shield against deleterious mutations

The vast majority of our DNA has an unknown function, and this mystery has led to debate between different scientific teams. One camp believes most DNA is indeed 'junk', with little to no important function. The other believes there are essential functions which are yet to be fully established. 

In 2012 a large group called ENCODE released a paper suggesting that 80% of our DNA was in fact functional, while this was somewhat liberally defined. The latest entry on the subject, published in Genome Biology and Evolution, has now put forward that 75% of our DNA is largely non-functional aside from the important function of protecting functional DNA by taking up the majority of new mutations. 

“For 80% of the human genome to be functional, each couple in the world would have to beget on average 15 children and all but two would have to die or fail to reproduce”

Most of the content of our chromosomes may simply function as a protective mechanisms against rogue mutations. Credit: Andreas Bolzer, Gregor Kreth, Irina Solovei, Daniela Koehler, Kaan Saracoglu, Christine Fauth, Stefan Müller, Roland Eils, Christoph Cremer, Michael R. Speicher, Thomas Cremer, via Wikimedia Commons

Most of the content of our chromosomes may simply function as a protective mechanisms against rogue mutations. Credit: Andreas Bolzer, Gregor Kreth, Irina Solovei, Daniela Koehler, Kaan Saracoglu, Christine Fauth, Stefan Müller, Roland Eils, Christoph Cremer, Michael R. Speicher, Thomas Cremer, via Wikimedia Commons

The study suggests that a large proportion of non-functional DNA is required in species with lower fertility rates. Given that mutations happen regularly, the chances of those mutations interfering with critical genes becomes much more unlikely if a small proportion of the genome is critical. The majority of mutations instead occur in non-functional DNA; keeping the mutation rate in important DNA relatively small. 

The scientists put forward that if more of our DNA was critical, it would require a much greater fertility rate to offset the likelihood of infant mortality and disease as a result of mutations in essential genes. The researchers postulate that 25 percent is likely to be the upper limit of functionality, but 10-15 percent is more realistic. If this is true, 'junk' DNA does have an important indirect function; shielding the genome from harm. The argument will probably continue for many years as we delve deeper into the human genome, but this theory is an interesting addition to the debate. 

“I would like to think that most knowledgeable biologists who properly appreciate evolutionary theory and genomic diversity are well aware of the many problems with ENCODE’s claim” 

Read more at The Scientist